In science-based disciplines, articles that report on original research studies are considered primary sources, and often referred to as empirical. Empirical studies are often published in peer-reviewed journals. These articles are usually divided into sections, including introduction, literature review, methods, data, results, discussion, conclusion / suggestions for further study, and references.
Articles that interpret or analyze original research studies are considered secondary sources, and often referred to as review articles. Examples include literature reviews, systematic reviews, and meta-analyses. Note that these articles may also be divided into sections. If there is a methods section, it will discuss how research articles (primary sources) were selected for review and analyzed.